1. Field of the Invention
The present invention relates to a printed circuit board, which is usually a flat board whose front contains slots for integrated circuit chips and connections for a variety of electronic components, and whose back is printed with electrically conductive pathways between the components.
2. Description of the Prior Art
Methods for soldering the leads of electronic components on the printed circuit boards are roughly classified into two groups. One is the screen printing method, in which the screen printing machine coats solder cream onto the land of the printed circuit board. The other is the precoat soldering method, in which the solder bump is formed on the land of the printed circuit board by means of an appropriate coating means using, for example, the solder plating.
The screen printing method is cheaper in cost, but is disadvantageous in that the adjustment of coating amount is so difficult that the solder cream cannot be coated uniformly. If too much solder cream is coated, the circuit tends to cause a short circuit because the excessive solder forms a bridge or a ball on the printed circuit board. On the contrary, if too little solder cream is coated, the lead cannot be surely secured onto the land.
Furthermore, the lead is becoming smaller and narrower as the technology advances. Accordingly, the size of the land on the printed circuit board is correspondingly becoming smaller and narrower. In view of such recent advancement of the technology, the screen printing method may not be a prospective soldering method when applied to the printed circuit board due to above-described disadvantages.
On the other hand, in the case of the precoat soldering method, the adjustment of the solder amount to be coated on the land can be easily and accurately done. Thus, the precoat soldering method is recently used for forming the solder bump on the land.
The rigidity of the lead is lowered as its size becomes smaller. Therefore, a smaller and narrower lead is likely to bend back. If the electronic component has a plurality of such smaller and narrower leads, some leads may deform upward. It will directly affect the installation of electronic component on the printed circuit board. Because, these leads may not be able to land on the solder bump of the land, when the electronic component is connected to the printed circuit board. In such a case, the electronic component will not be completely soldered onto the printed circuit board and may produce a defective, even if only one of plural leads deforms.
Moreover, the recent size reduction of the land apparently brings the height reduction of the solder bump on the land. It will increase the percentage of production defects.